1. Field of the Invention
The present invention relates to media sensors. More specifically, the present invention provides methods and arrangements for media edge sensors useful, for example, in a label printer.
2. Description of Related Art
Edge detection is used for identifying the passage of leading and or trailing edges of media as a means for counting and or accurate spatial registration of operations to be performed upon desired areas of the media. For example, label printers pass an array of labels releasably adhered to a support web past a printhead. An emitter and a detector pair are positioned on either side of the support web to detect changes in the web transmissivity between areas of the web covered by a label and the areas of uncovered web between each label. When the transmissivity changes from high to low or vice versa, a signal is transmitted to the printer processor indicating that a label edge has been detected. Thereby, accurate spatial orientation of printed indicia upon each label is enabled.
Some prior edge sensors have used an aperture to localize the emitter output and or mask the detector as a means for increasing the rate of change between a high transmissivity and a low transmissivity state, as a label edge passes the detector. As shown in FIG. 1, because of light scattering that occurs in the web, even if an aperture is used, a sharply defined transition does not occur. Noise generated in part by the presence of paper fibers or other non-uniformities in the web and or labels introduces a further random error to the detector by varying the point, relative to the actual edge location, at which a preset transition threshold signal level is detected.
The emitter, detector, aperture and their precise placement with respect to each other introduces further opportunity for variability of the sensor response characteristics. Performance characteristics of sensor components may vary batch to batch as the different components are received from a single or multiple suppliers and over time as component sensitivity and or output levels degrade. Further, environmental fouling of the emitter, aperture and or detector will degrade sensor circuit response characteristics over time.
Alternatively, edge detection may be performed by illuminating the back of the web and detecting the reflectivity changes caused by passage of, for example, a black mark placed on the back of the web, relative to a label edge. Black marks may also be used to indicate approach of a media run-out condition. However, reflectivity and diffusion variances in the web and or printed marks can still create similar signal response random error characteristics as noted above. Furthermore, different placements and performance characteristics of sensor components from batch to batch, and environmental fouling of such components over time, can also still degrade sensor circuit response characteristics.
Nonetheless, users expect label and other such printers and devices to function with a wide range of different media and support web combinations having a wide range of transmissivity and or light scattering characteristics. Therefore, it is an object of the present invention to provide methods and apparatuses that overcome such deficiencies in the prior art.